Three conditions for an object to be considered a ‘planet’

International Astronomical Union (IAU) voted in 2006 to update the definition of what makes a planet. According to their decision a planet must satisfy the following three criteria:

• It must be an object which independently orbits the Sun
• It must have enough mass so that gravity pulls it into a roughly speroidal shape
• It must be large enough to “dominate” its orbit (ie. its mass must be much larger than anything else which crosses its orbit

Source:  Curious about astronomy